The long-term storage of waste in landfills can be problematic for several reasons. First, landfill space is limited. On the other hand, the production of waste materials seems to be increasing at an ever-incredible pace. Moreover, conventional landfills can pose serious environmental problems including the contamination of nearby groundwater and the generation of air pollutants. Unfortunately, the treatment and recycling of most conventional forms of waste remains a serious challenge. As a minimum, an effective treatment/recycling solution must be energy efficient and present minimal environmental risks.
In general, conventional treatment/recycling schemes have been somewhat limited when applied to mixed wastestreams and wastestreams that include bulk solids. For example, consider a continuous feed, high pressure system for processing waste. The pressurized nature of these processes typically requires that bulk solids be ground to a fine particle size to allow the pumping of the particularized solids into a high pressure reactor. Both grinding and pumping can require specialty equipment. In particular, a different device is generally required for different materials such as wood, plastic, or friable solids. Once the material has been ground, introduction into a pressurized reactor usually requires slurrying the material at a high concentration to minimize the size of the reactor and associated process equipment. Thus, expensive, high pressure slurry pumps for viscous streams are typically required. For other solids such as metals, glass or ceramics, suitable size-reduction for introduction into a pressurized reactor vessel is generally impractical.
In addition to conventional wastestreams, a large amount of waste is generated each year that is hazardous and cannot be placed in a conventional landfill unless it is pre-treated. Among this hazardous waste is a large amount of mixed waste consisting of non-hazardous solids that are contaminated with hazardous constituents. Examples of such mixed wastes include soils, inorganic adsorbents and other solids that are contaminated with hazardous organic materials. Another such mixed waste consists of conventional and chemical munitions as well as munition dunnage. Protective suits, munition bodies and equipment contaminated with energetics, biological or chemical warfare agents is another mixed waste that cannot be safely placed in a conventional landfill without pretreatment. Similarly, PCB contaminated transformers, pesticide contaminated bags and containers, medical/biohazard waste such as contaminated needles and glass containers, and computer waste that can include lead and other hazardous materials are all mixed wastes that cannot be safely placed in a conventional landfill.
Another factor that must be considered when contemplating the treatment/recycling of materials is the generation of treatment by-products that can present handling difficulties and in some cases interfere with the treatment process. For example, when supercritical water oxidation (SCWO) type processes are used to treat wastestreams, sticky solids are often generated that can plug a reactor vessel, absent special precautions. In a similar manner, when low and moderate temperatures are used as part of a treatment process, organics that are present in a wastestream often generate tars which are difficult to handle and process. Similarly, partial oxidation gasification systems typically generate dirty process effluents and can be difficult to control because of variations in the heat capacity, water content, and reactivity of the wastes.
Perhaps the most important consideration when considering the treatment/recycling of waste is the energy required to process the waste. Depending on the process, significant amounts of energy may be required to heat the waste, pressurize or depressurize a reactor vessel, and/or mix and transport the waste. Heretofore, processes such as plasma arc pyrolysis and other electrically heated systems have typically required large amounts of power to heat and vaporize the waste. For this reason, these processes typically cannot generate net power and as a consequence have not been widely adopted.
In light of the above, it is an object of the present invention to provide systems and methods suitable for the purposes of efficiently treating feedstocks such as wastestreams which do not generate sticky solids or tars. It is another object of the present invention to provide systems and methods for treating feedstocks for the purpose of waste destruction, energy generation, or the production of useful chemicals. Yet another object of the present invention is to provide systems and methods for chemically converting feedstocks which are energy efficient, simple, and economical.